Well, I mean, I can think of several different variations of a stovetop version (bechamel, the sodium citrate, or the condensed milk version that's been gaining popularity), a question of whether to bake it or not, endless add-ins, choices of cheese to use, choice of pasta to use. Making a bechamel without lumps is not hard but it does require some knowledge of basic technique. And it is actually conceivable that someone would use fresh pasta, which is a whole different ballgame. So while macaroni and cheese in its most basic is not at all a difficult dish it leaves tons of room for putting a personal spin on it...which means it has plenty of ways it can go wrong, too. I've certainly had plenty of disappointing mac and cheese in my life, sometimes at restaurants that really should be able to do better.